[llvm-dwp] Fix endianness issues (#203424)
llvm-dwp fails on SystemZ, which is big-endian, as follows:
$ cat min.c
int main() {}
$ clang -g -O0 -gsplit-dwarf min.c -o min
$ llvm-dwp -e min -o min.dwp
error: compile unit exceeds .debug_info section range: 905969668 >= 58
This is because it hardcodes IsLittleEndian=true in multiple places. Fix
by forwarding endianness of the current object file.
Add a SystemZ-specific test.
Add proper big-endian support to X86/compress.test and add an
explanation regarding what the hardcoded blobs are and how they are
generated.
